adding/subtracting like terms

help me with this please.

i've got the basics but this is pretty strange.

(i) 2ba + 3ab = 5ab
(ii) 3ab + ba = 4ab
(iii) xy + 2yx = 3xy
(iv) 7bca - 5abc = 2abc

what i've noticed is that the answers seem to be in ascending order.. am i wrong? if so, is it also wrong to write in descending order?

the multiplication is commutative, so abc = cba = acb = cab and so on, so the order doesn't matter.
you could write 7bca - 5abc = b2ca if you want, it's just conventional to write the constant first and then variables in alphabetical order, like 2abc.

thanks >:)